Encapsulated resistor



June 9, 1954y w. J. RANDOLPH ENCAPSULATED -REsIsToR Filed Aug. 16, 1961INVENTR. Mu rf? J. ,Qq/vamp# BY 4m/yens United States Patent() t3,136,972 v ENCAPSULATED RESISTOR Walter J.'Randolph, Bedminster, NJ.,assignor to Consolidated Electronics Industries Corp., New York, N.Y., acorporation of Delaware Filed Aug. 16, 1961, Ser. No. 131,898

3 Claims. (Cl. SSS-273) This invention relates to encapsulated resistorsin which the resistive element' is a thin film on the outer surface ofan insulating body. The purpose of encapsulated resistors is to preventadverse environmental conditions, such as water vapor or other matter inthe air, from attacking the resistor and changing its resistance eitherpermanently or temporarily. Encapsulation of electrical components isusually accomplished by inserting the component into a suitablecontainer, frequently of the same material as the encapsulatingmaterial, and then filling the space around the component withencapsulating material in a liquid form. After the encapsulatingmaterial hardens, the component is effectively sealed in, and if this isproperly done, subsequent changes in the environment should have littleor no effect on the electrical value of the component.

The main problem in encapsulating resistors of the type in which theresistance element is a thin film on the surface of a ceramic body isthat, if the encapsulated resistor is subsequently heated and cooled,the difference between the coefficients of thermal expansion of theceramic and of the encapsulating material may squeeze or abrade the thinfilm and cause it to change resistance by a considerable amount.However, simply enclosing the resistor within a sleeve does notcompletely prevent undesired changes, and in accordance with the presentinvention it has been found necessary to coat the encapsulated elementswith a mold release which has the desired effect of rendering theencapsulated resistor almost entirely immune to external environment,including changes in the temperature of the component.

The'invention will be described in greater detail in connection with thedrawing in which the only figure is a crosssectional View of anencapsulated resistor constructed according to the invention.

In the drawing a cylinder 11 of some suitable insulating material suchas porcelain or another known form of insulating material is coated witha thin, resistive film 12. The resistive film 12 is. usually ofcarbonaceous material and may be helically grooved in accordance withknown techniques to increase the accuracy of pre-setting of theresistance and for other purposes whichy are well known. A pair of metalend caps 13 and 14 attached to opposite ends of the insulating body 11covers the ends of the resistive film 12 to make connection therewith,and a pair of wire terminals 16 and 17 is attached to the end caps 13and 14, respectively, to extend therefrom. A coatingrofvarnish 18 isapplied to the resistor to cover the resistive film itself and toprevent any changes therein and to cover the end caps 13 and 14 and asmall portion of the adjacent wire terminals 16 and 17. y Prior toencapsulating the resistor just described, a sleeve 19 of encapsulaingmaterial is slipped over the end caps 13 and 14. This sleeve ispreferably made of an irradiated polyolefin which has the characteristicof shrinking to a predetermined diameter when heated to a predeterminedtemperature. It also has exceptionally good resistance to tearing.

After the sleeve 19 is slipped over the end caps, the whole thing isheated to a temperature which will cause the sleeve to shrink down and`grasp the end caps 13 and 14 tightly and thereby seal off a space 20immediately surrounding the critical area between the end caps. Be-

3,136,972 Patented June 9, 1964 cause the amount of shrinkage can bepredetermined, it

`caps 13 and 14, or more properly, the outer surface of the thin layerof varnish thereover.

Following the shaping of the sleeve 19, both the sleeve and a shortsection of the terminals 16 and 17 are coated with a layer 21 of moldrelease'. For this purpose, I have found it advantageous to dip thesleeved unit into a suitable silicone mold release. It is convenient atthis time to bend the terminal 16 back more or less parallel to theterminal 17 so that these two terminals mayL then be inserted throughapertures 22 and 23, respectively, in the bottom of an open-toppedreceptacle 24, This receptacle may be made of any suitable insulatingmaterial, such as one of the epoxy resins used widely for encapsulationpurposes. Thereafter, the remaining space in the receptacle 24 is filledwith an encapsulating material, and, particularly if the receptacle ismade of an epoxy resin, it is desirable to fill the receptacle with moreepoxy resin material in a liquid form. This completes the encapsulationprocess and forms a monolithic mass with only the two terminals 16 and17 extending from it. By properly spacing the apertures 22 and 23, thedistance between the terminals 16 and 17 may be set to a figurecommensurate with established spacings for printed circuits.

While it has been attempted heretofore to encapsulate film resistorswithout the layer 21 of mold release, it has been found that theencapsulating material clings to the film 12 or to the varnish 18. Theaddition of the mold release overcomes this problem and permitssatisfactory encapsulation of such resistors.

While this invention has been described in limited terms as it relatesto a specific embodiment, it will be understood by thse skilled in theart that the true scope is determined by the following claims and thatmodifications may therefore be made from the specific embodimentdescribed without departing from the scope of the invention.

What is claimed is: f y

1. An encapsulated resistor comprising: an insulating support cylinder;a resistive film coated on the cylindrical surface thereof; a pair ofend caps attached to the ends of said cylinder and making electricalconnection with the ends of said resistive film; a pair of wireterminals extending from said caps; a thermally shrunk insulating sleevesurrounding the cylindrical surfaces of said support cylinder and saidend caps, the inner surface of said sleeve being shrung to a diameterslightly greater than the diameter of said resistive film so as not tomake mechanical contact with said film; a mold release coating coveringthe outersurface of saidsleeve and the exposed ends of said caps; ahollow container for said resistor, said container comprising a hollowcylindrical body and an integral end having two holes therein throughwhich said wire terminals extend; and epoxy filler substantially fillingthe remaining space within said container and encasing said resistor.

2. An encapsulated resistor comprising: an insulating support cylinder;a resistive film coated on the outer cylin drical surface thereof; apair of end caps attached to the ends of said cylinder and makingelectrical connection with the ends of said resistive film; a pair ofwire terminals extending from said caps; a thermally shrunk sleeve ofirradiated polyolefin material surrounding the cylindrical surfaces ofsaid support cylinder and said end caps and extending therebeyond, theinner surface of said sleeve being shrunk to a diameter less than thediametetr of said caps and slightly greater than the diameter of saidresistive film so as not to make mechanical contact with said film; amold release coating covering the outer surface of said sleeve and theexposed ends of said caps; a hollow epoxy container for said resistor,said container support cylinder; a resistive lilm coated on thecylindrical surface thereof; a pair of endcaps attached to the ends ofsaid cylinder and making electrical connection with the ends of saidresistive film; a pair of wire terminals extending from said caps inopposite directions and substantially parallel to the axis of saidsupport cylinder, one ofsaid wire terminals being bent back on itselfand extending approximately parallel to the other wire terminal and insubstantially the same direction; a thin varnish coating over said capsand the portion of said lrn therebetween; a thermally shrunk insulatingsleeve surrounding the cylindrical surfaces of said support cylinder and`said end caps and extending therebeyond, the inner suri face of saidsleeve being shrunk to a diameter less than the diameter of said capsand slightly greater than the diameter of said resistive iilm so as notto make mechanical contact with said film; a mold release coatingcovering the outer surface of said sleeve and said varnish coating onthe exposedends of said caps; a hollow epoxy container for saidresistor, said container comprising a hollow cylindrical body and anintegral end plate having two holes therein through which said Wireterminals extend; and epoxy filler substantially filling the remainingspace within said container and encasing said resistor.

References Cited in the tile of this patent UNITED STATES PATENTS2,130,156 Rollefson Sept. 13, 1938 2,664,487 Gorden Dec. 29, 19532,685,016 Blackburn July 27, 1954 2,742,551 Kohring Apr. 17, 19562,803,729 Kohring Aug. 20, 1957

1. AN ENCAPSULATED RESISTOR COMPRISING: AN INSULATING SUPPORT CYLINDER;A RESISTIVE FILM COATED ON THE CYLINDRICAL SURFACE THEREOF; A PAIR OFEND CAPS ATTACHED TO THE ENDS OF SAID CYLINDER AND MAKING ELECTRICALCONNECTION WITH THE ENDS OF SAID RESISTIVE FILM; A PAIR OF WIRETERMINALS EXTENDING FROM SAID CAPS; A THERMALLY SHRUNK INSULATING SLEEVESURROUNDING THE CYLINDRICAL SURFACES OF SAID SUPPORT CYLINDER AND SAIDEND CAPS, THE INNER SURFACE OF SAID SLEEVE BEING SHRUNG TO A DIAMETERSLIGHTLY GREATER THAN THE DIAMETER OF SAID RESISTIVE FILM SO AS NOT TOMAKE MECHANICAL CONTACT WITH SAID FILM; A MOLD RELEASE COATING COVERINGTHE OUTER SURFACE OF SAID SLEEVE AND THE EXPOSED ENDS OF SAID CAPS; AHOLLOW CONTAINER FOR SAID RESISTOR, SAID CONTAINER COMPRISING A HOLLOWCYLINDRICAL BODY AND AN INTEGRAL END HAVING TWO HOLES THEREIN THROUGHWHICH SAID WIRE TERMINALS EXTEND; AND EPOXY FILLER SUBSTANTIALLY FILLINGTHE REMAINING SPACE WITHIN SAID CONTAINER AND ENCASING SAID RESISTOR.